One month out, the battle for control of the U.S. Senate is as tight as it's ever been. Democrats need to essentially take over four seats to win back the majority this November. They have one chance in six.

But those numbers don't reveal the rough ride it's been so far for Democrats. In Ohio, Sen. Rob Portman (R) appears to be pulling away with his race, so this week it comes off our list of 10 states most likely to flip parties. Florida looks less like a pickup opportunity for them, too. And in the open seat in Indiana, Rep. Todd Young (R) is closing a polling gap against the well-funded, well-known politician Evan Bayh (D).

Democrats managed to raise their chances as races in some red-leaning states (Missouri and North Carolina, in particular), become more competitive. This week, we changed both from "lean Republican" to "toss-up."

Taken together, in battle for the Senate, Democrats have a narrow edge. Which means of these 10 races is worth watching. Here are the latest top 10 most likely to flip parties, ranked from least to most likely. To the line!

10. Arizona (Republican-held): Arizona was on our list earlier this year, then it got knocked off by Indiana. But Sen. John McCain's attempt at a sixth term has always been one to watch, thanks in large part to the corner Trump has put him in. His centrist challenger, Rep. Ann Kirkpatrick (D), is raising the money to be competitive ($3 million in the third quarter, which is a record for a statewide Democrat in Arizona) and is focused on tying McCain to Trump, especially on immigration. Right now, McCain has an average 16-point lead in a RealClearPolitics average of polls. But Democrats say the Latino vote will break late, and we could see a world in which this race would come down to whether they can get minorities out to vote in big enough numbers to make a difference. (Previous ranking: None in a few months.)

9. Florida (R): Sen. Marco Rubio (R) may have his flaws (like, saying he wouldn't run for Senate, then running for Senate), but Democratic challenger Rep. Patrick Murphy's attacks saying as much just don't seem to be gaining traction. Senate Democrats' campaign arm canceled ads in this expensive state earlier this week; a sign they, too, aren't holding out hope Rubio can be toppled. (Previous ranking: 8)

8. Missouri (R): That Missouri is more competitive is one of the more surprising changes to the Senate map a month out from Election Day. Democrats trying to unseat Sen. Roy Blunt (R) are helped by Blunt's slow-starting campaign and a savvy candidate in Missouri Secretary of State Jason Kander, who has run what politicos say is hands down the best ad this election cycle of any candidate. But Missouri is still a state that trends red at the federal level -- Blunt has led in pretty much every major poll -- and one Trump is expected to win. (Previous ranking: 10)

7. North Carolina (R): Polls show this race narrowing, which is good news for Democrats. As my colleague Aaron Blake wrote earlier this week, of the 10 high-quality polls conducted in September, Sen. Richard Burr (R-N.C.) led in five, his Democratic challenger, Deborah Ross, led in four, and the 10th was a tie. Ross is helped out by the fact that Team Clinton is pouring in money and staff to get out the vote in the urban areas of the state, and Democrats are confident the former ACLU lawyer can weather some of Burr's recent attacks against her. (Previous ranking: 7) 

6. Indiana (OPEN): This race in a Trump-leaning state came on the radar at the last possible minute for Democrats, but suddenly the path to victory doesn't seem as clear-cut.  With Sen. Dan Coats (R) retiring, former governor and senator Evan Bayh decided in July that he wanted his old seat back and was willing to spend some $10 million to do it. Republicans immediately hit Bayh with attacks that he doesn't live in the state anymore -- he was registered as an inactive voter in Indiana. Outside groups came in to spend for Rep. Todd Young (R), cutting down Bayh's financial advantage and, it seems, some of his popularity. A new WTHR/HPI Indiana poll (taken with a grain of salt, since the firm polled for Young in the primary) has the race in a dead heat. It also shows Bayh's net favorability rating dropping 16 points in September, from 48 percent favorable/28 percent unfavorable, to 45 percent favorable / 39 percent unfavorable. Democrats say as long as Bayh remains more popular than not, he should be fine. (Previous ranking: 4)

5. New Hampshire (R): Sen. Kelly Ayotte (R) is up. Then Gov. Maggie Hassan (D) is up. The next day it's Ayotte's turn to have the momentum. This race between two popular politicians is as tight as they come, but will that change after Ayotte said in a debate Monday that "absolutely" Trump would be a good role model, then had to walk it back? (A Boston Globe/Suffolk University poll taken in the days before and after suggest no, showing Ayotte with a six-point lead.) Right now Clinton has a five-point lead in the state, according to the average of polls, and analysis by The Washington Post in September and Roll Call in October suggests Ayotte is positioned to survive a Trump loss by that margin. She's outperforming the nominee that has given her so much heartburn by an average eight points. (Previous ranking: 3)

4. Nevada (OPEN): Nevada is another razor-thin race, but Rep Joe Heck (R) has led against retiring Senate Minority Leader Harry Reid's (D) hand-picked opponent, former state attorney general Catherine Cortez Masto, in nearly every single public poll. That's not an easy feat in the year of Trump in a diverse swing state against a well oiled Democratic machine. We're still not counting Team Reid out, since close polls in Nevada can be deceiving; in the past, Hispanic voters have broken late and overwhelmingly for Democrats. (Previous ranking: 6)

3. Pennsylvania (R): We caveat that races 3 through 7 could get jostled around and probably still accurately reflect the map -- and we have jostled most of them over the past few months. This time it's Pennsylvania's turn to be one of the most competitive, mostly because it's the swing state on this list where Clinton has the biggest edge. Right now Clinton has a six-point lead according to a recent RealClearPolitics polling average. Polls here for the Senate race are swinging wildly in either direction, but the bottom line is Sen. Pat Toomey (R) appears to be hanging on over Democrat Katie McGinty in spite of Trump, and what Democrats feel are Toomey's deal-breaker comments on abortion. But can an incumbent outperform Trump by six points? That's much less clear. (Previous ranking: 5)

2. Wisconsin (R): Absent any major shake-up, Wisconsin remains one of Democrats' best pickup hopes. Much like Toomey in Pennsylvania, Sen. Ron Johnson (R) could run a perfect campaign and still get knocked out in this state that leans blue in presidential years. Right now, Clinton is up by five points in the RealClearPolitics average, and Senate Democrats' campaign arm just canceled ads in this state, suggesting they think former senator Russ Feingold (D) won't have trouble closing. (Previous ranking: 2) 

1. Illinois (R): Illinois remains our top flip possibility, but don't count Sen Mark Kirk (R) totally out. His campaign is going on the offensive against Rep. Tammy Duckworth (D), suggesting she didn't listen to whistleblowers when she was head of the Illinois Department of Veterans Affairs. There have been a handful of polls suggesting this race is within the margin of error, and we'll be watching to see if Kirk's offensive moves the numbers in a more steady direction in his favor. (Previous ranking: 1)